Abstract

Abstract. Mineral dust deposition is suggested to be a significant atmospheric supply pathway of bioavailable iron (Fe) to Fe-depleted surface oceans. In this study, mineral dust and dissolved Fe (Fed) deposition rates are predicted for March 2009 to February 2010 using the 3-D chemical transport model GEOS-Chem implemented with a comprehensive dust-Fe dissolution scheme. The model simulates Fed production during the atmospheric transport of mineral dust, taking into account inorganic and organic (oxalate)-promoted Fe dissolution processes, photochemical redox cycling between ferric (Fe(III)) and ferrous (Fe(II)) forms of Fe, dissolution of three different Fe-containing minerals (hematite, goethite, and aluminosilicates), and detailed mineralogy of wind-blown dust from the major desert regions. Our calculations suggest that during the year-long simulation ~0.26 Tg (1 Tg = 1012 g) of Fed was deposited to global oceanic regions. Compared to simulations only taking into account proton-promoted Fe dissolution, the addition of oxalate and Fe(II)/Fe(III) redox cycling to the dust-Fe mobilization scheme increased total annual model-predicted Fed deposition to global oceanic regions by ~75%. The implementation of Fe(II)/Fe(III) photochemical redox cycling in the model also allows for the distinction between different oxidation states of deposited Fed. Our calculations suggest that during the daytime, large fractions of Fed deposited to the global oceans is likely to be in Fe(II) form, while nocturnal fluxes of Fed are largely in Fe(III) form. Model sensitivity simulations suggest Fed fluxes to the oceans can range from ~50% reduction to ~150% increase associated with the uncertainty in Fe-containing minerals commonly found in dust particles. This study indicates that Fed deposition to the oceans is controlled by total dust-Fe mass concentrations, mineralogy, the surface area of dust particles, atmospheric chemical composition, cloud processing, and meteorological parameters and exhibits complex and spatiotemporally variable patterns. Our study suggests that the explicit model representation of individual processes leading to Fed production within mineral dust are needed to improve the understanding of the atmospheric Fe cycle, and quantify the effect of dust-Fe on ocean biological productivity, carbon cycle, and climate.

Highlights

  • Aeolian dust deposition has been suggested to be a critical source of the micronutrient iron (Fe) to ∼ 30 % of the world’s oceans, known as high nitrate-low-chlorophyll (HNLC) regions where marine primary productivity can be limited by the supply of Fe (Martin and Fitzwater, 1988; Coale et al, 1996; Boyd et al, 2000)

  • As the objective of this study is to estimate the supply of bioavailable lithogenic aerosol Fe to different regions of the global oceans through atmospheric pathways, here we only consider dissolved Fe (Fed) production within mineral dust during its atmospheric transport and do not take into account marine processes leading to the formation of bioavailable Fe

  • In order to determine the accuracy of Goddard Earth Observing System (GEOS)-Chem/Fed-predicted surface oxalate concentrations, model-predicted values are compared to a global dataset of oxalate concentration measurements (Myriokefalitakis et al, 2011)

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Summary

Introduction

Aeolian dust deposition has been suggested to be a critical source of the micronutrient iron (Fe) to ∼ 30 % of the world’s oceans, known as high nitrate-low-chlorophyll (HNLC) regions where marine primary productivity can be limited by the supply of Fe (Martin and Fitzwater, 1988; Coale et al, 1996; Boyd et al, 2000). It is further proposed that the supply of Fe may limit nitrogen fixation, exerting an important control on the primary productivity in vast areas of the global oceans (Falkowski, 1997; Mills et al, 2004). The atmospheric supply of Fe to the surface oceans may play a key role in regulating biological productivity, atmospheric carbon dioxide (CO2) concentrations, and possibly climate (Martin, 1990; Zhuang et al, 1992; Jickells et al, 2005).

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